[12.x] Clarify behavior of --hours in queue:flush command

[amirhshokri]

Why?

While we generally avoid documenting all options for every command, the --hours behavior for queue:flush is different from --hours in queue:prune-failed, which could easily confuse developers.

• queue:flush --hours=N deletes jobs at or before N hours ago.
• queue:prune-failed --hours=N deletes jobs older than N hours.

Because of this subtle difference, it is important to clarify it in the docs to prevent unexpected deletions.

Changes

• Added a short section to the queue:flush command documentation highlighting that --hours exists and explaining its inclusive behavior.
• Included an example for clarity.